Laser drilling of aircraft parts uses a high energy laser beam to remove material from the part, leaving a hole of a controlled diameter. If the part has a closed cavity, as is typical in airfoil blades, the energy beam must drill through the first surface but must be turned off before it strikes the second surface. The first surface penetration is called breakthrough, and while breakthrough is required, the energy beam must be turned off before damage to the second surface occurs.
Typically, a machine control is programmed to control the number of laser pulses or shots. Due to variations inherent in the manufacture of undrilled parts, such as wall thickness variation caused by casting changes or other features, the thickness of metal walls in parts is not always constant. Hence, the pre-programmed number of shots to break through the surface may be too few (leading to incomplete holes) or too many (leading to material scarring in the second surface). An underdrilled hole requires redrilling of the part, and an overdrilled hole results in added expense for material evaluation, reinspection, and sometimes part scrapping.
Various prior art methods have been developed for detecting a breakthrough condition before damage to the second surface occurs. For example, the workpiece may be viewed from the front or beam side with a radiation sensing system. However, in the operation of a pulse laser drill, the laser pulse beam striking the material to be drilled results in the production of very high temperatures by intense local heating in as much as laser drilling is a thermal process. The intense local heating causes very hot minute particles of material to be expelled from the localized drilling area. The heated minute particles, in the form of a plume of sparks, radiate heat and light, i.e., extrinsic radiation. When a radiation sensing system looking for intrinsic laser pulse radiation is used to determine if a hole is completely through, the extrinsic radiation from the hot minute particles causes false readings in the radiation system.
U.S. Pat. No. . 5,026,964, totally incorporated herein by reference, solves for the above-mentioned radiation sensing problem by providing a detection method capable of discriminating between intrinsic radiation and extrinsic radiation. However, the apparatus of the '964 patent uses a manually adjustable threshold that has to be individually set up for different laser voltages.
It is an object of the present invention to provide a method and apparatus for detecting when a through bore is drilled in a workpiece, without requiring manual adjustment for different laser voltages.
It is a further object of the present invention to provide such a method and apparatus which prevents further drilling once a through bore is drilled in the workpiece.
It is yet another object of the present invention which prevents underdrilling in a pulse laser drilling operation.